1. Field of the Invention
This invention relates to circuits used for comparing voltages, and in particular to a voltage comparator circuit for use in the pin electronics circuits of automatic test equipment.
2. Description of the Prior Art
In automatic test equipment for the testing of integrated circuits, a plurality of pin electronics interface circuits are coupled to the pins or other nodes of an electronic device being tested. Through these pins, stimuli signals are supplied to the device under test, and output signals from the device under test are detected and measured. Usually, the stimuli signals represent logic states or analog voltages or currents which are desired to be impressed on the pins of the device under test as a parallel pattern, with the resulting output signals checked in parallel.
The interface circuits function as an interface between the computer controlling the test system and the individual pins of the device under test. The interface circuits receive reference voltages and digital data from other circuits in the test system, and then through a driving switch these voltages or data onto the desired pin of the device under test under control of a program stored in the test system computer. Correspondingly, the interface circuits receive reference voltages or data from the device under test and supply it to a comparator circuit which compares the signal received with the proper response stored in the program of the test system computer. In such apparatus voltage comparators are frequently used to determine the potential of one or more pins of the device being tested relative to one or more reference potentials.
As integrated circuits have increased in speed, size, and complexity, the demands made on the voltage comparators in automatic test equipment have correspondingly increased. With the advent of commercially available automatic test equipment operating from a 50 mega-Hertz clock signal, improved voltage comparators are necessary to enable testing of the device under test at these high clock rates.
In the prior art, high voltage comparators relied upon balanced differential pairs of field effect transistors for detecting the relative magnitude of a reference voltage with an unknown voltage. Unfortunately, such devices are not suitable for detecting failures of short duration such as those found when testing emitter coupled logic devices.
In other prior art voltage comparison circuits, fast low voltage comparators were feasible, but could not handle the voltage swings of TTL and MOS logic families. Typical prior art voltage comparators are shown in: "Sentry.RTM. High Speed and Clock Driver Pin Electronics Reference Manual" available from Fairchild Test Systems Group, San Jose, California.